The disclosures of Japanese Patent Applications No. 2000-035796 filed on Feb. 14, 2000 and No. 2000-280218 filed on Sep. 14, 2000 each including the specification, drawing and abstract are incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a diagnostic apparatus for a fuel vapor purge system for use in an internal combustion engine installed in a motor vehicle such as an automobile.
2. Discussion of Related Art
In a conventional fuel vapor purge system installed in a vehicle, fuel vapor generated in a fuel tank is introduced through a fuel vapor conduit into a canister and trapped therein, and the fuel vapor thus trapped is then discharged (purged) when appropriate from the canister into an intake passage through a purge passage while the ambient air is being introduced into the canister.
Apparatuses for diagnosing a failure in the fuel vapor purge system, i.e., those for detecting leakage due to a hole(s) formed in a purge path, are also well known in the art. The purge path may include the fuel tank, fuel vapor conduit, canister, and purge passage. In many of the apparatuses, a negative pressure in the intake passage produced during operation of the internal combustion engine is introduced into the purge path through the purge passage, and then the purge path is temporarily sealed. In this condition, subsequent changes in the pressure within the purge path are measured by a pressure sensor. If the rate of increase of the pressure is higher than a predetermined value, the fuel vapor purge system is diagnosed as being at fault.
In order to accurately determine a failure in the fuel vapor purge system, the amount of fuel vapor generated in the fuel tank must be within a predetermined range. This is because the failure diagnosis of the fuel vapor purge system is conducted by sealing the purge path in the negative pressure state and detecting changes in the pressure in the purge path with time. Namely, if the amount of fuel vapor generated in the fuel tank is not within the predetermined range, it cannot be determined whether the pressure within the purge path has been raised by the ambient air introduced into the purge path through a hole(s) or crack(s) formed therein, or by a large amount of fuel vapor generated in the fuel tank.
In view of the above problem, a diagnostic apparatus as disclosed in Japanese Laid-open Patent Publication No. HEI 6-74104 is adapted to first detect leakage by introducing a negative pressure into a purge path, seal the purge path after introducing the atmospheric pressure into the path, and then measure the amount of fuel vapor generated in the fuel tank by means of the pressure sensor. The apparatus then corrects the result of leakage detection based on the result of measurement of the fuel vapor amount.
The outputs of the pressure sensor used in the above-described fuel vapor purge system involve errors caused by manufacturing variations, and the use of pressure sensors having output errors within a predetermined range of xe2x88x920.133 kPa (=xe2x88x921 mmHg)) to +0.133 kPa (=+1 mmHg)) in the atmospheric pressure state is allowed. In the diagnostic apparatus as described above, since the purge path that has been held in the negative pressure state is brought into the atmospheric pressure state, the minimum value of the above-mentioned output error range is determined as representing the atmospheric pressure state. In other words, when the output of the pressure sensor reaches xe2x88x920.133 kPa from the negative pressure side, it is determined that the purge path is in the atmospheric pressure state. The reason for this is as follows: in the case where the apparatus is designed to determine the atmospheric pressure state when the output of the pressure sensor is 0 kPa, and where the purge path suffers from leakage and almost no fuel vapor is generated in the fuel tank, the output of a pressure sensor that outputs the minimum value of the output error range is maintained at xe2x88x920.133 kPa. In this state, the diagnostic apparatus determines that the purge path has not yet reached the atmospheric pressure, and is thus inhibited from measuring the amount of fuel vapor generated.
For a pressure sensor that outputs the maximum value of the output error range, on the contrary, the minimum value of the output error range represents the state in which the internal pressure of the purge path is in the course of increasing toward the atmospheric pressure. If the purge path is sealed when the output is equal to the minimum value and the fuel vapor amount is measured, therefore, the measurement accuracy of the fuel vapor amount may deteriorate, resulting in a reduced accuracy in the leak diagnosis of the purge path.
It is therefore an object of the invention to provide a diagnostic apparatus for a fuel vapor purge system, which is capable of measuring fuel vapor generated in a fuel tank with improved accuracy, irrespective of variations in the output error, thus assuring an improved accuracy in the leak diagnosis.
To accomplish the above and other objects, the invention provides a diagnostic apparatus for a fuel vapor purge system wherein fuel vapor generated in a fuel tank is trapped in a canister, and the fuel vapor trapped in the canister is purged into an intake passage of an internal combustion engine through a purge path including the fuel tank, which system comprises (1) a pressure sensor that measures a pressure of a space in the fuel tank, (2) a diagnosing unit that conducts leak diagnosis of the purge path based on a change in the pressure in the fuel tank and an amount of fuel vapor generated in the fuel tank, the change in the pressure being measured after sealing the purge path while providing a difference between inside pressure and outside pressure of the purge path, the amount of fuel vapor being measured after applying an atmospheric pressure to the purge path and sealing the purge path, and (3) a determining unit that determines that the purge path has reached an atmospheric pressure state upon detection of a predetermined state after starting application of the atmospheric pressure to the purge path. In this system, the diagnosing unit measures the amount of fuel vapor generated in the fuel tank when the determining unit determines that the purge path has reached the atmospheric pressure state.
In diagnosing a failure in the fuel vapor purge system, the purge path is brought into the atmospheric pressure state so that the amount of fuel vapor generated in the fuel tank can be measured. The diagnostic apparatus according to the invention sets in advance a particular state in which introduction of the atmospheric pressure is completed, and determines that the purge path has reached the atmospheric pressure state when the above particular state is detected. Thus, the diagnostic apparatus of the invention is able to determine the atmospheric pressure state of the purge path with high accuracy, irrespective of variations in output errors among individual pressure sensors, thus assuring improved accuracy in the leak diagnosis.